1. Field of the Invention
The present invention generally relates to digital signal processing of the type carried out in an image processor and, more particularly, to a method and apparatus for determination of an approximate standard deviation with computational steps that are easily implementable in simplified hardware.
2. Description of the Prior Art
Particularly in the area of image processing, and predominantly for the purpose of noise suppression, the use of "sigma filters" is known. The function of sigma filters is based on replacing a signal value of a pixel belonging to a video image by a determined signal value that is formed by averaging with adjacent signal values, whose intensity lies within a fixed standard deviation range (sigma range) around the intensity value of the output signal value. The basic idea of this filtration consists in the fact that noise disturbances occur with a Gaussian distribution, so that effective noise suppression is possible within the standard deviation range. If a greater intensity difference of an adjacent signal value from the output signal value exists, there is a high probability that this difference in intensity is not caused by noise but by a different image signal content. This signal value should therefore not be included in averaging.
A modified sigma filter is described in prior German Patent Application P 4314 980.4. An improvement in noise suppression is achieved by virtue of the fact that depending on the respective standard deviation (sigma), limiting values are established within which signal values for averaging are used. Above a certain standard deviation, a constant limiting value is used. This results in a quality improvement in a video image to be reproduced, wherein unnatural image impressions with plastic effects are avoided.
Standard deviation is defined as follows: ##EQU1## Here N is the number of pixels over which the averaging is conducted.
Calculation of the standard deviation requires a number of computational steps performed in a processor, preferably in a hardware circuit. Implementation in a hardware circuit requires a relatively large chip area and is therefore costly. During the performance of the computational operations in a processor, considerable computation time is required, which in many cases rules out real-time applications.